Prostacyclin (or PGI2) is a member of the family of lipid molecules known as eicosanoids. It is a potent vasodilator, antiproliferative, anti-thrombotic agent that mediates its effects as an agonist of the IP receptor. The IP receptor is a G-protein coupled receptor that, upon activation by prostacyclin, stimulates the formation of cyclic adenosine monophosphate (cAMP). Prostacyclin counteracts the vasoconstrictor and pro-thrombotic activity of endothelin.
Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by a progressive pulmonary vasculopathy leading to right ventricular hypertrophy. Exogenous administration of an agonist of the IP receptor has become an important strategy in the treatment of PAH. (See, e.g., Tuder et al., Am. J. Respir. Crit. Care. Med., 1999, 159: 1925-1932; Humbert et al, J. Am. Coll. Cardiol., 2004, 43:13 S-24S; Rosenzweig, Expert Opin. Emerging Drugs, 2006, 11:609-619; McLaughlin et al, Circulation, 2006, 114:1417-1431; Rosenkranz, Clin. Res. Cardiol., 2007, 96:527-541; Driscoll et al, Expert Opin. Pharmacother., 2008, 9:65-81.).
The prostacyclin analogue epoprostenol (flolan) is at least as effective as transplantation in terms of survival. Despite this, it is not used as frontline therapy due to significant tolerability, convenience and cost issues. Instead, patients with PAH are often treated first with either endothelin receptor antagonists (e.g. bosentan) and/or PDE5 inhibitors (e.g. sildenafil), which are better tolerated but can have limited efficacy. Prostacyclin analogues are used mainly as add-on treatment as severity of the disease progresses and tolerability and convenience become less of an issue.
Two key issues prevent current prostacyclin analogues being used as frontline therapy in PAH. Firstly, they are very unstable with an extremely short half-life, meaning they must be constantly infused via an in-dwelling intra venous (i.v.) catheter that is both inconvenient for the patient and also associated with a significant risk of infection and sepsis. Secondly, they are associated with significant side effects including nausea, jaw pain, headache and other side effects associated with systemic hypotension.
One solution to these issues is iloprost, which is available as a nebulised formulation that has reduced tolerability issues, but the short half life results in a 6-9 times daily dosing regime. More recently, researchers made efforts to generate stable, orally available IP receptor agonists. These ligands would improve patient convenience and compliance, but high levels of systemic drug is required to achieve pharmacodynamic effects in the lung; thus, possibly generating similar side effects to those observed with i.v. flolan.
The present invention describes stable, highly selective IP receptor agonists that are suitable for oral and inhaled delivery. The present invention offers a significant improvement over existing prostacyclin analogues and enables their use in less-severe patients. In addition, long term activation of the IP receptor has been shown to reverse remodeling associated with PAH; therefore, earlier intervention with the present invention may have significant effects on disease progression and potentially may show reversal.
In addition, pharmaceutical research has considerable interest in developing IP receptor agonists for the treatment of pulmonary fibrosis. IP deficient mice have been shown to be more susceptible to bleomycin-induced lung fibrosis than wild-type animals (Lovgren A K et al. (2006) Am J Physiol Lung Cell Mol. Physiol. 291:L144-56), and the IP receptor agonist iloprost increases survival in bleomycin-treated mice (Zhu et al (2010) Respir Res. 11(1):34).
Furthermore, IP receptor signaling has been shown to exert beneficial effects in fibrotic conditions of various organs in animal models and in patients. Benefits of IP receptor agonist were shown for fibrosis of the heart, lung, skin, pancreas and liver, and in systemic sclerosis. (Gayraud M (2007) Joint Bone Spine. 74(1):e1-8; Hirata Y et al (2009) Biomed Pharmacother. 63(10):781-6; Kaneshige T et al (2007) J Vet Med. Sci. 69(12):1271-6; Sahsivar M O et al (2009) Shock 32(5):498-502; Sato N et al (2010) Diabetes 59(4):1092-100; Shouval D S et al (2008) Clin Exp Rheumatol. 26(3 Suppl 49):5105-7; Spargias K et al (2009) Circulation. 120(18):1793-9; Stratton R et al (2001) J Clin Invest. 108(2):241-50; Takenaka M et al (2009) Prostaglandins Leukot Essent Fatty Acids. 80(5-6):263-7; Watanabe M et al (2009) Am J. Nephrol. 30(1):1-11; Yano T et al (2005) Am J Pathol. 166(5):1333-42; Zardi E M et al (2007) Expert Opin Biol Ther. 7(6):785-90; Zardi E M et al (2006) In Vivo 20(3):377-80; Rehberger P et al (2009) Acta Derm Venereol. 89(3):245-9). Fibrotic conditions can occur in most organs secondary to chronic inflammation indications throughout the body and are likely to share common causes. Therefore, antifibrotic agents such as IP receptor agonists of the present invention are of potential benefit in all indications that are associated with fibrotic tissue remodeling.
There is considerable interest in developing agonists of the IP receptor for use in the treatment of other diseases, such as atherothrombosis, preeclampsia. It is highly desirable to develop stable, inhaled agonists of the IP receptor, which may lead to improved management of PAH.